Media captionEngineers will now finesse Rosetta's trajectory for rendezvous with a comet

Rosetta, Europe's decade-long quest to put a robotic lander on a comet, has reached a key milestone.

The probe, which has spent the past two-and-half-years moving through space in a deep sleep, was expected to rouse itself at 10:00 GMT, ready to send a signal to Earth.

Receipt of this "I'm awake" message will confirm the great endeavour is still on course.

Rosetta is due to rendezvous with Comet 67P/Churyumov-Gerasimenko in August.

The despatch and landing of the small robot currently piggybacking the probe is set for November.

The reactivation of Rosetta is occurring some 800 million km from Earth, out near the orbit of the planet Jupiter.

Controllers at the European Space Agency's (Esa) operations centre here in Darmstadt, Germany, do not know precisely when Monday's all-important contact will be made, but they anticipate their consoles lighting up sometime between 17:30 and 18:30 GMT.

"It will be transmitting just the 'carrier signal', so at that point there's no data coming down from the spacecraft," explained Andrea Accomazzo, Rosetta's spacecraft operations manager.

"We just receive a firm frequency. In theory, it would be like a continuous beep if you were to convert it into sound.

"We will see it on a screen that is basically a spectrum analyser. Although, we will have no information from the spacecraft, we will know just from that transmission that it must have done everything it had to do automatically and is in a safe status; and that everything that happens next is in our hands," he told BBC News.

Rosetta must work through a sequence of pre-programmed activities before sending the signal. None of these activities has a fixed time length.

They include warming the spacecraft's navigation instruments, and finding Earth on the sky so that the probe's main communications antenna can be pointed in the right direction for the call home.

First contact is likely to come through the US space agency's 70m Goldstone radio dish in California.

If the signal does not arrive as expected, controllers will hold off any intervention until Tuesday morning.

The Darmstadt team can send commands to Rosetta that would forcibly bring it out of its slumber, but the preference is to give the probe sufficient time to complete the necessary tasks automatically.

Rosetta was put into hibernation in June 2011 because its trajectory through the Solar System was about to take it so far from the Sun that its solar panels would produce minimal power. The decision was therefore taken to put the spacecraft in a deep sleep.

Launched back in 2004, the probe has taken a rather circuitous route out to its comet target.

This has involved making a number of flybys of the inner planets, using their gravity to pick up sufficient speed for the eventual comet encounter.

It has already delivered some fascinating science, particularly from the close passes it made to two asteroids - the rocks Steins, in 2008, and Lutetia, in 2010.

Once controllers have a full assessment of the health of Rosetta, they will initiate a series of burns on its thrusters to close the gap to 67P. Currently at a separation of nine million km, this will be reduced to a mere 10km by mid-September.

The landing of the three-legged robot called Philae in November is sure to be a nail-biting event.

"I know that Nasa colleagues talked about 'seven minutes of terror' for the landing of the Curiosity rover on Mars; it'll be more like four hours of terror between the separation of Philae and its 'docking' on the comet," said Esa director general Jean-Jacques Dordain.

The intention is for Rosetta to follow the comet as it moves closer towards the Sun, monitoring the changes that take place on the body. Philae will report changes that occur at the surface.

Comets - giant "dirty snowballs", as some have called them - are believed to contain materials that have remained largely unchanged since the formation of the Solar System 4.6bn years ago, and Rosetta data should therefore help researchers understand better how our local space environment has evolved over time.

"Rosetta is a unique mission - unique technologically, unique scientifically, and unique philosophically because comets may be at the origin of who we are," Mr Dordain told BBC News.

Image copyrightAirbusImage caption
Philae robot's task in November: No mission has ever attempted to make a soft landing on a comet before